1. Field of the Invention
The present invention relates to an apparatus and method for manufacturing flat glass in which the glass is formed while being supported on the surface of a pool of molten metal. More particularly, this invention relates to an apparatus and method for introducing protective atmosphere gases into a headspace of the chamber in which the glass is formed.
2. Brief Description of the Prior Art
In a variety of processes a continuous sheet or ribbon of glass is formed while supported on and advancing along the surface of a pool of molten metal. In general, a protective nonoxidizing atmosphere is provided in a headspace above the advancing glass and the molten metal supporting it in order to prevent the oxidation of the molten metal and consequent contamination of the glass. Several patents illustrating the use of protective atmosphere to protect molten metal in this environment are U.S. Pat. No. 3,241,937 to Michalik et al; U.S. Pat. No. 3,337,322 to Taylor; U.S. Pat. No. 3,356,476 to Gulotta; U.S. Pat. No. 3,551,126 to Sacrez et al and U.S. Pat. No. 3,630,701 to Javaux et al. All of these patents show the introduction of protective atmosphere gases into a headspace overlying an advancing body of glass that is being formed into a continuous sheet and over a pool of molten metal that supports the advancing glass. The head-space into which the protective atmosphere gases are introduced is defined by the side walls and end walls of the forming chamber and by some overlying ceiling or roof spaced above the advancing glasses and its supporting metal.
As shown in U.S. Pat. No. 3,468,869 for example, electrical heating elements for controlling the temperature of the forming chamber may be mounted in the ceiling above the glass. The ceiling, which extends over the advancing glass and its supporting metal, may be separated from and enclosed within an outer shell or casing as shown in U.S. Pat. No. 3,721,543 to Classen et al. Such an arrangement is shown generally for electrically heated enclosures in U.S. Pat. No. 2,543,708 to Rice et al. The patent of Rice et al shows a double-walled furnace having an inner wall like the ceiling of a glass forming chamber made of refractory material surrounded by an outer metallic wall or casing. Electric heaters extend through the inner wall to maintain the temperature within the interior space.
According to the disclosure of Rice et al, a suitable gaseous mixture for introduction into the inside chamber defined by the inner wall is introduced into the space between the inner and outer walls and allowed to diffuse through the inner wall into the furnace. While passing through the space between the inner and outer walls, such a gas inherently removes some heat from the electric heaters and their connections to conductors located within the space between the inner wall and outer wall of the disclosed furnace or chamber.
Employment of a gas introduction method, such as disclosed by Rice et al, in a chamber for forming glass results in the introduction of atmosphere gases throughout the headspace of the forming chamber dependent upon the porosity of the ceiling extending over that headspace and separating it from an outer shell or casing surrounding the chamber. To the extent that the introduction of cool atmosphere gases into the space between the ceiling and the casing serves to cool electrical connections serving heaters which extend through the ceiling, the temperature of that space at various locations adjacent such electrical connections dictates the amount of atmosphere gases that must be introduced in order to maintain electrical connections at suitable operating temperatures. To the extent that atmosphere gases must be introduced to protect the molten metal within the chamber from oxidation at various locations along the length of the forming chamber, conditions within the headspace of the chamber itself dictate the amount of atmosphere gases which must be introduced into the chamber and the relative concentrations of protective gases which must be introduced. The two objectives of maintaining the temperature of electrical connections and maintaining suitable protective gas conditions within the headspace of the chamber are not fully compatible. It is commonly necessary to introduce an excess of atmosphere gases and an excess of reducing gases within the total flow of atmosphere gases in order to provide sufficient thermal and oxidation protection within the chamber. The present invention provides an apparatus and method for avoiding compromise of the two objectives for introducing atmosphere gases into a glass forming chamber.